1. The field of art to which the invention pertains includes measuring and testing art as directed to pressure gauges in which the operating mechanism is completely immersed in a dampening fluid that fills the gauge casing.
2. Pressure gauges and the like containing an oil fill have been widely used and are commercially available from a variety of manufacturing sources. Specific reasons for selecting or requiring an oil filled gauge versus a dry or oil unfilled gauge vary, but generally are attributed to either protecting the working mechanism against corrosion and/or system vibration or pulsation to which the instrument is subjected. Exemplifying liquid filled pressure gauges of the prior art are the disclosures of U.S. Pat. Nos. 3,335,609; 3,370,470; 3,776,041 and 3,874,241.
While such gauges of the prior art are particularly suitable in the applications for which they are expressly intended, a problem which has plagued their use is the instrument error which can result from volumetric expansion of the liquid fill with increases in temperature. That is, any increase (or decrease) in fluid pressure internally of the enclosure caused by volume changes from ambient temperature variations tends to act externally against the Bourdon tube, pressure bellows or the like, tending to distort its operational span from that normally anticipated. This, in turn, results in a false readout of the instrument despite the features of accuracy otherwise built in by the manufacturer. The extent of error per degree temperature change will, of course, vary as a function of various factors such as pressure range of the gauge, rigidity of the enclosure components, effective fluid volume, volumetric temperature coefficient of expansion of the fill fluid, etc. In pressure ranges generally operable below 200 psig, this error can become most significant amounting to as much as 30 percent for a plus 70.degree. F. temperature increase on a 15 psig pressure gauge.
For overcoming this adverse pressure effect of temperature induced expansion various approaches have been proposed, including the use of a controlled air space, elastomeric bladders, diaphragms, etc. contained internally of the enclosure to accommodate volumetric changes with a minimum of pressure buildup. Common to these approaches has been an oversized enclosure required to accommodate the expanded volume of the fluid and/or the additional components intended therefor. Associated with these approaches is the added expense which they almost invariably involve. Despite recognition of the foregoing, a ready solution to the problem has not heretofore been known.